There I was, 29km in the air, putting torches everywhere on a floating island. I have More Overlays installed so I can tell that it is properly lit up. I saw a bunch of hostile mobs (skeletons, creepers, zombies etc.) and had not seen them there before. I set it to Peaceful mode, waited for them to vanish, then back to Normal mode and they started appearing on the other side of the island (also well lit).

I will try and see if it happens with all extraneous mods removed.

This is strange. Did it happen in earlier versions?

Update: I removed all extraneous mods, set to peaceful, went to Y=2570, spawned a 101x101 platform, placed torches and set to normal/night. Moved away from center and BAM, mobs! I'm using build 298 and have no idea if it's in earlier versions as well.